Method and device for optically measuring distance or speed

ABSTRACT

The invention relates to a method for measuring a distance-related variable such as distance, speed or acceleration, or for levelling. An emitting device ( 1 - 6 ) comprising at least one microlaser ( 3 ) is used as an emitter, and the beam reflected off an object surface is received and evaluated for measurement. A beam is emitted and evaluated after being received, in a substantially simultaneous manner in at least two different wavelengths. Preferably, said method is carried out by means of a device whereby the microlaser ( 3 ) in the form of a passively pulsating microlaser ( 3 ) emits at least two different wavelengths. A receiver ( 15, 40 ) for the laser beam reflected off an object is arranged downstream from the evaluating device ( 41 ).

BACKGROUND OF THE INVENTION

The present invention relates to a method for optically measuringdistance or speed, according to the preamble of claim 1, and to a devicefor optically measuring distance or speed or for a levelling device,according to the preamble of claim 4.

In devices for measuring distance and/or speed, the range—with otherwiseidentical technical properties—is the greater the higher the peak pulsepower of the emitted laser pulses. On the other hand, for safetyreasons, the peak pulse power cannot be increased without limit. Thelaser safety regulations permit higher peak pulse powers forwavelengths >1050 nm, i.e. in an invisible wavelength range, than forvisible radiation.

Although this makes it possible to achieve longer ranges, a visiblelaser beam also has a number of advantages, such as

-   -   visualization of the measuring beam on the object;    -   possibility for checking and, if required, adjusting the        position of the axis of the laser beam relative to the telescope        axis;    -   reduction of the danger to the eye through the aversion reaction        of an affected person, including the blink reflex.

SUMMARY OF THE INVENTION

It is the object of the invention to provide a method and a device ofsaid type so that all stated advantages are achievable. This object isachieved by the defining features of claim claim 1 and claim 4,respectively.

When it is stated that the at least two different wavelengths of theradiation are evaluated after being received, this need not mean thatboth lead to a result, but it is possible for the visible beam to beevaluated, for example, so that it is used for marking the object or thetarget point, whereas the other wavelength, for example in the infraredrange, serves for a distance measurement or a variable derivedtherefrom, such as speed or acceleration. This will generally be atransit time measurement, in particular with pulsed light, more rarelywith phase comparison, although the distance measurement could also beperformed trigonometrically, i.e. in the manner of a basicdistance-measuring apparatus, as in the case of levelling devices.

In each case, however, it is thus possible, for example, to correct theeffect of atmospheric refraction as well as to combine the advantages ofa visible laser beam with those of a laser in the infrared range.

The output of at least two different wavelengths could be effected perse in different ways, for example by arranging two light sources whichare preferably (but not necessarily) combined with one another in asingle beam path. However, it will generally be preferable if the laserarrangement has only a single laser for generating the at least twowavelengths.

It is advantageous if a device operating according to such a method hasthe features of claim 4. This is because a passively pulsatingsolid-state laser can be used. However, conventional solid-state lasersare very energy-consumptive, so that limits are encountered here if theyare to be supplied with sufficient energy in a portable device, such asa distance-measuring device. The use of a microlaser, as described, forexample, by N. Mermilliod et al. in Applied Physics Letters, 1991, Vol.59, No. 27, page 3519, makes it possible for a passively pulsating laseralso to be advantageously used in a portable device.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details and features of the present invention are evident fromthe following description of embodiments shown schematically in thedrawing.

FIG. 1 shows a first embodiment of a tacheometer optical systemaccording to the invention;

FIG. 2 shows the available pulses of two different wavelengths;

FIG. 3 shows a second embodiment of a tacheometer optical systemaccording to the invention and

FIG. 4 shows a hand-held device designed according to the invention, formeasuring distance.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Initially a pump laser 1 with a convex lens 2 arranged in front of itand a thermoelectric cooler 1′ present behind it is provided in atachometer housing T merely indicated by a dash-dot line. The pump laser1 throws, in a manner known per se, pump power in the form of radiationonto a microlaser 3 which is present in front of it in the focal planeof the lens 2 and in turn emits a laser beam via a lens 4. It isadvantageous if a block filter 5 for the pump laser radiation of thelaser 1 is coordinated with this (individual or multiple) lens 4, inparticular present downstream of said lens in the manner shown.

In the beam path, an optical frequency converter 6 is provided after themicrolaser (or microchip laser) 3. With a frequency converter, such as,for example, KTP (potassium titanyl phosphate) as a frequency doubler, awavelength of about 532 nm, which is in the visible range, is generatedby generating the second harmonic, in addition to the wavelength outputby the laser 3, for example of about 1064 nm (infrared). “Additionally”because such an optical frequency converter converts only a part of theincident radiation. Two different wavelengths λ1 and λ2 therefore emergesimultaneously at the exit of the frequency converter 6, as shown inFIG. 2.

Where an optical “frequency converter” is referred to here, and afrequency doubler is the preferred form of such a frequency converter,frequency tripling is also entirely possible. Such tripling can beeffected by frequency doubling and subsequent mixing (frequencyaddition) of the doubled signal with the signal of the laser 3, i.e. f

2 f, 2 f+f =3 f.

The emitted beam thus generated and having the two different wavelengthsλ1 and λ2 is deflected by a reflecting surface, such as a mirror 7. Abeam splitter 23 which guides a part of the transmitted beam to amonitoring means 23′ described further below is preferably arrangedthereafter. Consequently, correct and reliable operation of the opticalcomponents described above can be monitored in order to avoid damage inparticular by the invisible laser beam in the event of failure of thevisible radiation.

As shown in FIG. 1, further optical components 8 to 11 are provided inthe divergent beam path, i.e. in the emitted beam path. These comprisean optical system which produces beam divergence and can be formed, forexample, by a single lens 8. This lens is optionally part of atemperature compensator known per se, the lens holder not shown here ora spacer ring connected thereto consisting of a material having acoefficient of thermal expansion such that temperature-related focaldeviations are automatically compensated. For this purpose, the lens 8is displaced owing to the thermal expansion of such a ring, for exampleagainst the action of a spring pressing it against the ring, although aninterlocking connection of the lens with the expanding ring would alsobe conceivable. Such temperature compensators are known per se foroptical systems.

A first parallel plate 9 which can be mounted in an inclinable mannernot shown and with the aid of which the axis of the emitted beam and itsposition relative to the telescope axis A can be adjusted by incliningthe first parallel plate 9 so that it lies exactly along the telescopeaxis can be provided thereafter in this divergent emitted beam path. Forthis purpose, the parallel plate 9 can have a cardanic mounting and canbe adjustable with respect to its inclination to the axis A by anadjusting means known per se for optical purposes, such as two camsengaging the plate 9, acting at right angles to one another androtatable from the outside of the housing T.

If desired, a further optical member which can be switched on and is inthe form of a second parallel plate 10 acting as an attenuator andintended for defocusing the emitter during measurement in the case ofmeasurement to a reflector, in particular mounted on the object, can beprovided in this divergent emitted beam path, as indicated by an arrowa.

It has been found that the small divergence of less than 0.1 mrad,optionally of <0.05 mrad, or of only about 0.03 mrad, of the emitteddiffraction-limited beam during measurement to reflectors, such as, inparticular, triple reflectors, can be disadvantageous. This is because,on the one hand, the sighting of the reflector is very sensitive andtherefore not very convenient for the user of the device and, on theother hand, the turbulence in the atmosphere, the so-called “atmosphericflickering”, leads to strong variations in the measured signal and,consequently, to undesirably long measuring times. If a plane-parallelplate 10 is introduced into the beam path, the divergence of the emittedbeam is slightly increased, for example from 0.03 to 0.5 mrad.Preferably a blocking filter for the visible wavelength and anattenuating filter for the IR wavelength should be mounted on theswitchable parallel plate 10 so that the operator at the reflector isnot dazzled by the visible light and not harmed by the IR radiation.

If required, it is also possible to provide a switching means, ifnecessary in the form of the plane-parallel plate 10 itself, in orderalternately to emit an infrared or visible radiation component. Althoughthis increases the measuring time, the limits of the laser safetyregulations can be utilized fully, i.e. 100%, in this way (sequentialoperation). However, because the blink reflex of the eye takes 0.25 s,the change must be carried out relatively rapidly here, for examplefaster than 3 Hz, in particular faster than 4 Hz.

Furthermore, a telescope lens 18 is arranged in the tacheometer housingT. It will be possible per se to provide a separate emitting lensinstead of the deflecting mirror 7 as the linear continuation of thebeam emitted by the laser 3. If, however, the axis of the emitted beamis coaxial with the telescope axis A, according to the arrangementshown, parallaxes with respect to the field of view will be avoided. Asdescribed further below, the telescope lens 18 is preferably also thereceiving optical system, at least in the main.

The telescope lens has a chromatic longitudinal deviation or aberration.This will be a problem particularly when—as in the present case—at leasttwo different wavelengths λ1, λ2 are to be emitted and the lens 18 issimultaneously the emitting lens. A compensation lens 11 forcompensating this chromatic longitudinal deviation should thereforepreferably be provided in the emitted beam path. In the embodimentshown, this lens is composed of three members, for example cemented toone another.

In order to be able to survey object surfaces having a reflectivitydiffering from place to place and depth structures having a smalllateral dimension or having sharp edges and corners, the measuring spotmust be small and must not have troublesome diffraction rings. Beamshaving a Gaussian lateral intensity distribution in the short range (atthe lens 18) have no diffraction rings at long range (e.g. at 500 m) ifthe Gaussian beam is not cut off.

However, the Gaussian beam is of course cut off by the edge of the lens18. However, the intensity of the diffraction rings is the smaller thesmaller the chosen diameter of the Gaussian beam in the lens 18. Inpractice, it was found that the illumination of the lens 18 isadvantageously chosen so that the Gaussian laser beam is cut off by theedge of the lens only at an intensity value <0.09 times the maximumintensity. This is the case when the 1/e² beam diameter is =<0.9 timesthe lens diameter. In fact, the intensity of the first diffraction ringat long range (e.g. at 500 m) is then <0.03 of the maximum intensity,which is just sufficiently small for most cases in practice. The 1/e²beam diameter at the lens 18 should therefore be as small as possible sothat the intensity of the diffraction rings is small (e is Euler'snumber). On the other hand, as mentioned above, it is necessary for thecentral laser spot at long range (e.g. at 500 m) to remain as small aspossible. However, this is precisely the case when the laser beamdiameter in the lens 18 is large. In practice, the 1/e² diameter at longrange (e.g. at 500 m) should be not more than twice as large as theminimum possible diameter (Airy diffraction disk) which occurs onsimultaneous illumination of the lens 18. The central spot at long rangeis not more than twice as large as the Airy disk precisely when theillumination of the lens is chosen so that the edge of the lens cuts offthe Gaussian intensity distribution at an intensity >0.004 of themaximum intensity. This corresponds to a 1/e² diameter of the Gaussianbeam, which is 0.6 times the lens diameter.

A dimensioning method for illuminating the emitting lens is obtained asa compromise:

${{Lens}\mspace{14mu}{{diameter} \cdot 0.6}} < {\frac{1}{e^{2}} - {{diameter}\mspace{14mu}{of}\mspace{14mu}{laser}}} < {{lens}\mspace{14mu}{{diameter} \cdot {0.9.}}}$

As an example:

${\frac{1}{e^{2}} - {{diameter}\mspace{14mu}{of}\mspace{14mu}{laser}}} = {{lens}\mspace{14mu}{{diameter} \cdot {0.75.}}}$

Lens diameter 2a = 40 mm 1/e² diameter of the laser at the lens 2w₀ = 30mm 1/e² diameter of the laser at 500 m 2w₀ = 26 mm Diameter of the 1stdiffraction ring at 500 m 2r₁ = 52 mm Intensity ratio of 1st diffractionring to maximum intensity I₁/I₀ = 0.0008

The emitted beam path arriving from the compensation lens 11 isreflected by means of a splitter prism 12 having a partly reflectingdeflection surface 12′ into the beam path of the telescope lens 18 withthe axis A. The splitter prism 12 expediently has a further partlyreflecting deflection surface 12″ which is at a different angle and bymeans of which the received beam thrown onto it by the surface 12′ isguided along the axis A″ to a receiving means 13-15 and 34-40. Thesurface 12′ is expediently provided with a selective coating so thatonly the visible component of the received light is let through but theinfrared component is reflected.

The receiving means 13-15 and 34-40 has, in its entrance part, thereceiving surface of an optical fiber 15. A parallel plate isadvantageously present as a support of an antireflection screen 13, infront of this optical fiber 15. In the optical system shown, falsereflection occurs even with high-quality manufacture and treatment, thestrongest reflection of the emitted radiation which can be incident onthe receiving means 13-15 and 34-40 originating from the splitter prism12 itself, in particular from the surface 12″′. This reflected beam isof course divergent and has, at the position of the receiving fiber 15,a diameter which is about 500 times greater than the diameter of theoptical fiber 15 itself. On the other hand, that beam of the receivingradiation required for transit time measurement which is incident on theoptical fiber 15 is convergent owing to the focusing by the lens 18.This means that the diameter of the received beam will be smaller thanthe entrance area of the optical fiber 15, for example for an objectwhich is more than about 80 m away.

The problem of false reflection is solved by an antireflection screen 13which is arranged a distance away from the optical fiber 15, i.e.outside the image plane of the lens 18, which lies on the entrancesurface. Thus, reflected radiation is shaded off over the diameter ofthe entrance area of the optical fiber 15, while the received radiationincident on the entrance area of the optical fiber 15 is virtuallyunattenuated since it is still sufficiently wide at the position of theantireflection screen 13. Of course, reflections other than thoseoriginating from the surface 12″′ are also correspondingly attenuated bysuch an antireflection screen 13, since—in a centered opticalsystem—they lie on the axis of the received beam. Since the telescopelens 18 is a centered optical system, and since both the laser radiationsource and the entrance area of the optical fiber 15 are adjusted to theaxis of this lens, all reflections and the shadow of the antireflectionscreen 13 which is caused by the reflection lie along this axis.

As an example: Diameter of the antireflection screen: 0.6 mm Distance ofthe antireflection screen from   7 mm the entrance area of the opticalfiber Diameter of the received beam in the plane 2.3 mm of theantireflection screen Signal loss due to the effect of the 7%antireflection screen

Further false and control reflections are “swallowed” by a neutral glassplate 17 which acts as an optical sump and is provided at the splitterprism 12.

An attenuating element 14 can alternatively be switched into thereceived beam path by means of a final control element 14′ and servesfor attenuating the excessively large received signal at smalldistances.

Components 19 to 22 of conventional nature are parts of the telescope.They comprise a focusing lens system 19 which is formed from anachromatic lens and can be brought from one extreme position E1 shownwith solid lines into another extreme position E2 shown with dashedlines. This focusing lens system 19 is followed by an inverting prism 20for inverting the image obtained from the lens 18. It is then expedientto provide a reticule 21, for example having cross-hairs, and finally aneyepiece system 22 of conventional design.

By means of the beam splitter 23, a specific fraction of the emittedbeam is passed to a monitoring means 23′ which has at least one sensoror receiver 31 sensitive to the visible radiation emitted by theemitting arrangement 1 to 6. Thus, if the laser 3 is put into operationby means of a switch S, a start signal is sent to a logical stage 23 awhich can be addressed as a reference stage but is optionally formed byan NAND gate. In the present embodiment, this logical stage 23 a isformed by a flip-flop which is switched by the signal arriving fromswitch S in that the flip-flop 23 a is switched to its output Q. Thisoutput Q is connected to a control stage 23 b for the switch S and opensthe switch S when it receives a signal from the flip-flop 23 a. Thelaser 3 is thus switched off.

The other input of the flip-flop 23 a is connected to a pulse shaperstage 31′ which is connected to the receiver 31 for visible light andevaluates its signal. A corresponding filter 30 is present upstream ofthe receiver 31, unless said receiver itself is formed so that itresponds only to visible radiation. The receiver 31 receives the lightemitted by the emitting arrangement 1-6 via a deflecting mirror which—asexplained further below—is in the form of a beam splitter 28. Ifnecessary, an attenuating element 29, such as a screen or a filter, maybe present upstream of the receiver 31.

If—as a result of the incidence of visible radiation—the receiver 31receives a signal, the stage 31′ switches the flip-flop 23 a back to theoutput Q which is not connected to any downstream stage. In this case,the control stage 23 b therefore receives no signal and the switch Sremains closed until the main switch HS is manually operated. If an NANDgate is used, the result is a signal at the control stage 23 b wheneveronly the signal from the switch S is present at the input of the NANDgate, whereas the signal from the receiver 31 is missing.

It is advantageous if an indicator, for example by means of an acousticindicating device 1 a and/or a visual indicating device 1 v is connectedto the control stage for such a malfunction. A visual indicator canpreferably be in the form of an LCD display, in particular when—asexplained below—more than one possible error is monitored.

This is because this is only one possibility for monitoring the emittingarrangement 1 to 6. If the visible radiation is absent in spite of thefact that a switch is switched on, this would indicate a failure, forexample of the frequency converter 6 producing the visible radiation. Inthis case, there is the danger that persons will be affected by theinfrared laser beam without also being affected by the visible radiationattracting their attention, which is why the laser 3 should be switchedoff immediately.

The monitoring means 23′ can, however, also have further components. Itcan have a further infrared-sensitive receiver 27 which is preferablyupstream of the receiver 31 and monitors the emission of infraredradiation from the emitting arrangement 1-6. The circuit connected tothis receiver or detector 27 can be designed analogously to the circuitdescribed with reference to the stages 23 a, 23 b and 31′. Likewise, theoptical arrangement may once again have a filter 26, an attenuator 25and a beam splitter 24.

If in fact no signal at all is received after the switch S is closed,something which can be detected at the receiver 27, this does not mean,for example, that the frequency converter 6 is out of operation but thatthe laser 3 and/or its pump light source 1 is not functioning. Sincethis is a further monitorable malfunction, it is advantageous, asmentioned above, to form the visual indicator Iv as an LCD display. Inthis case, it will be advantageous if the control circuit 23 b is in theform of a processor which receives the output signals of the tworeceivers 31 and 27 directly and indirectly (via evaluation or pulseshaper stages, such as the stage 31′) and accordingly calls up thecorresponding indicator from memory with downstream driver stage foractuating the display.

The further part-beam obtained via the beam splitter 28 is fed via afurther attenuating element 32 (the power of the laser 3 shouldexpediently be attenuated before the sensitive components) and acollecting lens 33 to a further receiving optical fiber 34 or a furtherreceiver. This receiver serves for triggering the time counting in adownstream evaluation means 41 of conventional design.

The optical start pulse is advantageously fed via the same photodiode 40(or another optoelectrical converter such as a phototransistor) whichalso receives the light signal obtained via the fiber bundle 15 afterreflection by the sighted object.

A collecting lens 36 is present downstream of the fiber bundle 34 inorder to avoid any light losses. Thereafter, the start pulse lightpasses to a beam splitter surface 37 which advantageously emits thelight pulse via a filter 38 and further lens 39 to the photodiode 40.The filter 38 preferably transmits only the infrared laser radiation butblocks visible radiation, in order to prevent any influences. Thephotodiode 40 is part of the downstream electronic evaluation means 41.

The feed of the received pulse via the optical fiber 15 is formedanalogously thereto. The received light emerging from the optical fiber15 passes through a collecting lens 35 and then the beam splitter 27.

The visible component of the laser beam can be used for checking whetherthe position of the axis of the laser beam emitted by the emitting means1-6 corresponds to the telescope sighting axis A, since—as shown in FIG.2—the two emitted wavelengths λ1 and λ2 are coaxial, narrow light beamsso that the target point for the invisible wavelength component is alsodetectable on the basis of the visible light mark.

For such a check, it is advantageous to provide a mounting means, suchas a rail 42, but optionally also a pivotable support, for a triplereflector 11′ and to fasten it to the outside of the holder (not shown)of the lens 18. Thus, the triple reflector 11′ (or an arrangement ofmirrors or lenses performing the same function) can thus be moved fromthe rest position x to the operating position o in the axis A.Alternatively, the holder of the lens 18 has a thread or a bayonetfitting for pushing on or screwing on such a triple reflector or itsequivalent.

FIG. 3 shows a diagram corresponding to FIG. 1 (without electroniccomponents described above). Parts having the same function have thesame reference numerals as in FIG. 1. What is different in particular isthat a relatively thin beam splitter plate or parallel plate 16 can beswiveled into the convergent beam path behind the telescope lens 18. Thereason for this is that the combined splitter prism 12 for the emittedand the received beam path must be provided with a very low-reflectioncoating in the beam path of the telescope 18 in order to prevent thereceiver from being overloaded by the photodiode 40 by emitterreflections.

In order to use the splitter prism 12 only for inputting the emittedradiation and not also for outputting the received radiation (in whichcase the partial reflective coating of the surface 12″ may optionally bedispensed with), the parallel plate can be swiveled about a pivot pointP out of the rest position shown with solid lines into the operatingposition which is shown by dashed lines and in which it deflects thereceived beam along the receiving axis A″. Of course, instead of beingpivotable, the plate 16 could also be brought in a different manner froma rest position to the operating position shown, for example said platecould be displaceable.

Using an inclined parallel plate of this type in the convergent beampath of the telescope lens 18 has not been customary to date since thisresults in an astigmatism which adversely affects the quality of theimage. However, the magnitude of the astigmatism depends on therespective thickness of such a parallel plate 16. It is thereforepreferable if the parallel plate 16 has a thickness of <0.3 mm,preferably <0.2 mm, i.e. is very thin. For mechanical strengthening, itis therefore expediently supported by a frame surrounding it.

The parallel plate 16 is preferably coated so that the visible light forthe telescope function is transmitted and the appropriate splitter ratiois realized for the wavelength of the distance-measuring device (e.g.infrared): although it should also be conceivable, with regard to thepivotability of the parallel plate 16 (preferably toward a stop whichdetermines the operating position and is adjustable by a cam or a screwor the like), completely to dispense with a splitter function and toswivel the parallel plate 16 into the operating position for themeasurement and to dispense with viewing of the sighted object duringthis time, which is not very convenient for the user. In such a case,the parallel plate 16 could also be fixed. In the interest of improvedimage quality in the case of a parallel plate 16 present in the restposition, however, the pivotable arrangement is preferable.

While the embodiments according to FIGS. 1 and 3 have optical designs asare possible for a tacheometer, FIG. 4 shows the arrangement for aportable distance-measuring apparatus. It should be noted that whetherthe distance is determined by means of a single measurement or whetherthe speed or another derivative, such as the acceleration, is measuredby double measurement, depends only on the type of evaluation means 41(cf. FIGS. 1 and 3). It has also already been mentioned that theinvention can also be advantageously used in the case of levellingdevices based on the trigonometrical principle—for example forcorrecting atmospheric refraction. Such a levelling device could inprinciple have a design similar to that described below with referenceto FIG. 4. Here too, parts having the same function as in the precedingembodiments have the same reference numerals.

FIG. 4 shows the pump laser 1 on a Peltier element 1″. Microlaser 3,lens 4, frequency converter (KTP) 6, filter 5, mirror 7 and beamsplitter 23 are shown. A pin diode 48 for the reference light path ispresent on the holder 18′ of the lens, which in this case need notnecessarily be in the form of a telescope. The receiving lens is housedin a housing 43. The fiber holder with the fiber leading to aninterference filter and the receiving diode is merely indicated on theright of FIG. 4; in principle, this arrangement has a similar design tothat described above.

In the context of the invention, the design of the distance-measuringapparatus is of course variable within wide limits. The presentinvention makes it possible to provide a microlaser as a light source,radiation of at least two different wavelengths being emitted,optionally both via one microlaser, which are optionally emittedsimultaneously or substantially simultaneously (if the sequentialoperation described above following relatively rapidly in succession isdesired), and, after reception, the information available via theradiation of different wavelengths being evaluated together, either forthe actual measurement or for the sighting or the like. “Substantiallysimultaneously” is to be understood in this context.

It could also be possible, for example, to use two or more differentwavelengths in the invisible range for the correction of the atmosphericrefraction, the visible radiation optionally being dispensed with.

1. A method for optically measuring distance or speed, comprising:providing a transmitting arrangement having at least one microchip laserthat is adapted to emit radiation in the form of pulses and a frequencyconverter that converts a portion of the radiation emitted by said atleast one microchip laser to a different wavelength, wherein saidtransmitting arrangement emits radiation in the form of a common beamtoward an object, wherein said common beam being a diffraction-limitedGaussian beam of the at least two wavelengths (λ1, λ2), the common beamof the radiation reflected by a surface of an object being received andevaluated, wherein the transmitting arrangement emits at least radiationhaving a wavelength in the invisible range (λ1), said wavelength in theinvisible range and radiation of another wavelength (λ2) simultaneouslywith the radiation having a wavelength (λ1) wherein the furtherradiation of another wavelength (λ2) is in the visible range; evaluatingtogether the information available after the at least two wavelengths(λ1, λ2) have been received; and monitoring the radiation of visibleradiation and switching off at least that part of the transmittingarrangement which emits the invisible radiation if said monitoringdetermines that the visible radiation is absent.
 2. A device foroptically measuring distance or speed or for a levelling device,comprising: a transmitting arrangement emitting a laser beam via anobject lens; a receiver for the laser beam reflected by an object andpassed through said object lens; wherein said transmitting arrangementcomprises at least one microchip laser that is adapted to emit radiationin the form of pulses and a frequency converter, said frequencyconverter adapted to convert a portion of the radiation emitted by saidat least one microchip laser to a different wavelength; saidtransmitting arrangement emitting a common beam made up of at least twodifferent wavelengths (λ1, λ2) emitted simultaneously, one of saidwavelengths in the infrared range and one of said wavelengths in thevisible range, said beam being a diffraction-limited Gaussian beam ofdifferent wavelengths (λ1, λ2); said lens being formed for emission ofthe diffraction-limited, Gaussian beam; and evaluating means downstreamof said receiver for evaluating the at least two different wavelengths(λ1, λ2); and monitoring means for monitoring the radiation of visibleradiation and a switching stage for switching off at least that part ofsaid transmitting arrangement which emits the invisible radiation inresponse to said monitoring means determining that visible radiation isabsent.
 3. The device as claimed in claim 2, which is in the form of atachometer having a telescope lens.
 4. The device as claimed in claim 3,wherein the beam path of said tachometer into which the emitted beam ofsaid transmitting arrangement can be introduced is the beam path of saidtelescope lens.
 5. The device as claimed in claim 2, which is in theform of a hand-held distance-measuring apparatus or speed-measuringapparatus.
 6. The device as claimed in claim 2, wherein the lens isformed in such a way that the laser beam having a Gaussian intensitydistribution is vignetted and is cut off at an intensity value for afirst diffraction ring of <0.15 times a maximum intensity.
 7. The deviceas claimed in claim 6, wherein the intensity distribution of saidGaussian beam is cut off at an intensity value for the first diffractionring of less than 0.12 times.
 8. The device as claimed in claim 6,wherein the intensity distribution of said Gaussian beam is cut off atan intensity value for the first diffraction ring of less than 0.09times.
 9. The device as claimed in claim 2 including an optical filterwhich transmits only the invisible wavelength component but blocksvisible radiation upstream of the receiver.
 10. The device as claimed inclaim 2, wherein said transmitting arrangement and said receiver have acoaxial, common optical system.
 11. The device as claimed in claim 10including a tachometer with a telescope lens, said telescope lensforming said common optical system.
 12. The device as claimed in claim 2including a mounting means for an upstream arrangement of an opticalelement that is coordinated with the lens.
 13. The device as claimed inclaim 12, wherein said optical element comprises a back-reflectionelement.
 14. The device as claimed in claim 13, wherein said opticalelement comprises a triple reflector or a lens-mirror combination. 15.The device as claimed in claim 2 including an adjusting means foradjusting the laser spot in the divergent beam path that is coordinatedwith said lens.
 16. The device as claimed in claim 2 including acompensating lens for compensating the chromatic longitudinal deviationor aberration of the lens that is coordinated with said lens in anemitted beam path.
 17. The device as claimed in claim 2 including atemperature compensator that is displaceable as a function oftemperature and coordinated with said lens in an emitted beam path. 18.The device as claimed in claim 2 including an optionally switchableparallel plate for widening the beam that is coordinated with said lensin an emitted beam path.
 19. A device for optically measuring distanceor speed or for a levelling device, comprising: a transmittingarrangement emitting a laser beam via an object lens; a receiver for thelaser beam reflected by an object and passed through said object lens;wherein said transmitting arrangement comprises at least one microchiplaser that is adapted to emit radiation in the form of pulses and afrequency converter, said frequency converter adapted to convert aportion of the radiation emitted by said at least one microchip laser toa different wavelength; said transmitting arrangement emitting a commonbeam made up of at least two different wavelengths (λ1, λ2) emittedsimultaneously, one of said wavelengths in the infrared range and one ofsaid wavelengths in the visible range, said beam being adiffraction-limited Gaussian beam of different wavelengths (λ1, λ2);said lens being formed for emission of the diffraction-limited, Gaussianbeam; and evaluating means downstream of said receiver for evaluatingthe at least two different wavelengths (λ1, λ2); and a beam splitter forseparating the two wavelength components, and an antireflection screenarranged outside an image plane of said receiver upstream of saidreceiver.
 20. The device as claimed in claim 19 including a parallelplate upstream of said receiver in a convergent beam path of said lensfor outputting the measuring beam reflected by an object toward saidreceiver.
 21. The device as claimed in claim 20, wherein said parallelplate has a thickness of <0.3 mm.
 22. The device as claimed in claim 21,wherein said parallel plate has a thickness of less than 0.2 mm.
 23. Thedevice as claimed in claim 21, wherein said parallel plate is partiallyreflecting.
 24. The device as claimed in claim 20, wherein said parallelplate is movable from a rest position outside the beam path of said lensinto an operating position within the beam path.
 25. The device asclaimed in claim 24, wherein said parallel plate is pivotable.
 26. Thedevice as claimed in claim 19, wherein said beam splitter comprises aprism.
 27. The device as claimed in claim 19, wherein saidantireflection screen is dimensioned according to a cross-section of anentry surface of said receiver.